1. Field of the Invention
The present invention relates to halftoning by error diffusion, in which error diffusion weights are changeable and preferably depend on output error or on image source data plus accumulated error.
2. Description of the Related Art
Error diffusion halftoning, as generally described in the seminal works by Floyd, Steinberg and Stucki, has become one of the most popular techniques for producing halftoned image output based on a continuous tone image input. Particularly in connection with computerized images, where each input pixel is represented by an 8-bit gray scale or 24-bit color value, error diffusion halftoning has been found to yield pleasing results in images printed with binary or limited level output printers, such as color ink jet printers and the like. Error diffusion tends to enhance edge sharpness and further tends to preserve fine image detail while yielding an overall pleasing image.
Generally speaking, error diffusion halftoning proceeds in accordance with the following steps. First, image data for a target pixel in the continuous tone image is compared against a threshold (or, in the case of multi-level output devices, against a range of thresholds), so as to determine what output value the image output device should print for the target pixel. For example, in the case of a binary printer (meaning a printer that outputs at each pixel location either a dot or an absence of a dot), the threshold may be one half of the density range, with a pixel being printed if the continuous tone value for the target pixel exceeds one half and with no pixel being printed if the continuous tone value does not exceed one half. Then, the error between the continuous tone input value and the actual output value is calculated. This error is sometimes called the "output error". The output error is diffused to adjacent pixels using predetermined weighting coefficients, so that predesignated proportions of the output error are added to existing continuous tone image values at pixels adjacent the target pixel. Processing then proceeds with another target pixel in a predetermined scan pattern. When it comes time to threshold one of the adjacent pixels so as to determine whether or not to print a dot, the determination is based on the original continuous tone image value plus any accumulated errors.
One problem with error diffusion halftoning is the creation of artifacts in the printed image that tend to degrade overall printed image quality. Two kinds of artifacts have been identified: unexpected continuous dot lines that appear in highlight regions (such as at extremely low and extremely high density regions), and repetitive textures or patterns that appear at mid density regions. These artifacts are illustrated in more detail in connection with FIGS. 1 and 2.
In FIG. 1, 10 represents a printout of standard error diffusion halftoning of an 8.times.4 grid of patches, with each patch having a constant gray level that increases serially from patch to patch. The 8.times.4 grid of FIG. 1 includes patches only in the low density gray region and does not include patches of darkness exceeding around ten percent gray. Continuous dot lines are readily apparent, for example, at 11 and 12. These continuous dot line patterns are displeasing to a viewer.
FIG. 2 illustrates a gray level wedge proceeding from a full black density level to a full white density level. At mid density levels, repetitive patterns or texture are readily apparent, such as illustrated at 16. These textures are displeasing to a viewer, and degrade overall printed image quality.
The creation and presence of these artifacts are well-documented, and have been subjected to significant study so as to reduce their appearance and improve overall image quality. One previously-proposed technique is to modify error diffusion weights based on the image input. Examples of such techniques are described in Ostromoukhov, European 808,055; and Eschbach, "Reduction Of Artifacts In Error Diffusion By Means Of Input-Dependent Weights", Journal of Electronic Imaging, October, 1993, pages 352 through 358. Other known techniques, although not necessarily known to reduce artifacts, include techniques that dynamically modify the threshold against which the continuous tone image value plus accumulated error is compared, including modification based on image value, as described for example in Eschbach, "Error-Diffusion Algorithm With Edge Enhancement", Journal of The Optical Society of America, December, 1991, pages 1844 through 1850.
Such techniques are different from the present invention, since in the invention error diffusion weights and/or thresholds are modified based on accumulated output error, or based on image source data and accumulated output error.